This invention relates generally to electrical machines and more particularly, to methods and apparatus for operating electrical machines.
At least some known electric power generators include a stator and a rotor coupled to an external excitation system that, in some configurations, includes a brushless exciter that includes a rotating armature and a diode rectifier. Specifically, at last some brushless exciters are electrically connected to a generator field winding coupled to the generator rotor. The rotating armature and diode rectifier are sometimes rotatably coupled to the generator rotor is a configuration in which the diode rectifier is configured as a diode wheel and wherein the rotating armature, the diode wheel, and the generator rotor are electrically connected. An alternating current (AC) signal generated within the rotating armature is transmitted to the diode wheel wherein the AC current signal is rectified to a direct current (DC) signal. The DC signal is transmitted to the generator field windings to facilitate the generation of electric power.
Some known diode wheels include a plurality of diodes that generally permit electrical current flow in one direction and inhibit current flow in the opposite direction. During periods when electrical current flow is permitted, an electrical signal with predetermined voltages and currents is transmitted and the diode is sometimes referred to as being in a conductive state. During periods when electrical current flow is inhibited, the diode is sometimes referred to as being in a non-conducting state. As the rectifying diode transitions from a conductive state to a non-conductive state, an electrical current and voltage excursion may be generated. More specifically, such a signal is sometimes referred to as a reverse recovery signal, and such voltage excursions are typically referred to as commutation voltage spikes. When the commutation voltage spikes are transmitted from the diodes to the generator field windings, a voltage threshold of electrical insulation associated with the rotor winding may be exceeded. Over time, continued exposure to voltage spikes may cause the insulation to breakdown. To facilitate reducing the transmission of voltage spikes to the rotor windings, some diode wheels include snubber circuits including capacitors. However, known capacitors may not have a sufficient response time characteristics and/or may be of such mass and size as to inhibit the operation of the diode wheel.